Steve Smith has been ruled out of the third Ashes Test through illness, with Usman Khawaja earning an immediate reprieve as Australia won the toss and batted first.

After missing training earlier this week due to sickness, Smith hit again on Wednesday morning before a lengthy discussion with coaching staff.

But he was then spotted leaving the ground, opening the door for Khawaja to come back into the team at No.4 just a day after his Test career appeared over.

"Steve is feeling a little bit unwell the last couple of days," captain Pat Cummins said.

"He came and gave it a crack this morning but didn't think he was going to get up for this one so he's headed off home.

"But we're pretty lucky we have got someone like Usman who can step right in."

Cummins has returned to Australia's side following his back issues, while Nathan Lyon is also back.

Seamers Michael Neser and Brendan Doggett will miss out, as announced on Tuesday.

Khawaja will turn 39 on day two of this Test, and had initially been overlooked for a return to the Test side after overcoming back spasms.

The decision for him to bat at No.4 allows Travis Head to stay at the top of the order with Jake Weatherald, as is Australia's preference.

The hosts can wrap up the series with victory in Adelaide, but the absence of Smith comes as a huge blow.

The 36-year-old remains Australia's most reliable batter, with scores of 61 not out and 23 in the eight-wicket win in Brisbane earlier this month.

Winning the toss, however, is a victory for the hosts.

Hot conditions are expected through the first two days, and England captain Ben Stokes had made no secret of the fact he intended to bat first if given the chance.

England, meanwhile, have made the one change with Josh Tongue coming in for Gus Atkinson.

Australia: Jake Weatherald, Travis Head, Marnus Labuschagne, Usman Khawaja, Cameron Green, Alex Carey, Josh Inglis, Pat Cummins (capt) Mitchell Starc, Nathan Lyon Scott Boland.

England: Zak Crawley, Ben Duckett, Ollie Pope, Joe Root, Harry Brook, Ben Stokes (capt), Jamie Smith, Will Jacks, Josh Tongue, Brydon Carse, Jofra Archer.